Micro-mixer/combustor

ABSTRACT

A micro-mixer/combustor to mix fuel and oxidant streams into combustible mixtures where flames resulting from combustion of the mixture can be sustained inside its combustion chamber is provided. The present design is particularly suitable for diffusion flames. In various aspects the present design mixes the fuel and oxidant streams prior to entering a combustion chamber. The combustion chamber is designed to prevent excess pressure to build up within the combustion chamber, which build up can cause instabilities in the flame. A restriction in the inlet to the combustion chamber from the mixing chamber forces the incoming streams to converge while introducing minor pressure drop. In one or more aspects, heat from combustion products exhausted from the combustion chamber may be used to provide heat to at least one of fuel passing through the fuel inlet channel, oxidant passing through the oxidant inlet channel, the mixing chamber, or the combustion chamber. In one or more aspects, an ignition strip may be positioned in the combustion chamber to sustain a flame without preheating.

CLAIM OF PRIORITY TO RELATED APPLICATION

This application claims priority to co-pending U.S. provisionalapplication entitled “MICRO-MIXER/COMBUSTOR” having Ser. No. 61/851,683,filed on Mar. 12, 2013, which is incorporated by reference as if fullyset forth herein.

CROSS-REFERENCE TO RELATED DOCUMENTS

This application makes reference to and incorporates by reference thefollowing paper as if it were fully set forth herein expressly in itsentirety:

J. Badra & A. R. Masri (2012): Design of a Numerical Microcombustor forDiffusion Flames, Combustion Science and Technology, 184:7-8, 1121-1134(attached hereto as Appendix B).

TECHNICAL FIELD

The present disclosure generally relates to micro-combustors, inparticular for diffusion flames as an energy source.

BACKGROUND

The need for replacing batteries was stimulated by work at theMassachusetts Institute of Technology (MIT) Gas Turbine Laboratory,whose researchers were amongst the first ones to fabricate aminiaturized gas turbine generator. A. H. Epstein & S. D. Senturia,Science 276 (1997) 1211. Since then, the designs of micro-combustorshave attracted attention.

Combustion is the rapid oxidation of fuels accompanied by the emissionof energy, usually in the form of heat and light. Combustion occurringwithin sub-millimeter volumes is known as micro-combustion. The releaseof heat can result in the production of light in the form of eitherglowing or a flame. The advantage of micro-combustors is their abilityto utilize hydrogen or hydrocarbon fuels which have extremely highspecific energies of approximately 142 MJ/kg and 45 MJ/kg, respectively.The best batteries currently available, lithium sulfur, have an energydensity of only 0.792 MJ/kg. Therefore, even if only 1% of the storedchemical energy of a hydrocarbon fuel were converted into useable powerits power output would be competitive with that of batteries. If theseefficiencies can be achieved, micro-combustion could lead to thedevelopment of power sources with high power-to-weight ratios. Thesewould be attractive electronic and electrochemical devices where a keyconsideration is the size and weight of the power source.Micro-combustion also presents other advantages over batteries in thatit could also reduce hazardous waste by eliminating battery productionand disposal, as micro-combustion devices are refueled, not replaced.

Attempts to design a micro-combustor, particularly for diffusion flames,have suffered from some basic but critical outstanding issues, includingfluid mixing and flame stability. The difficulty in flame stability isimposed by the small volume of the reactor and hence, the flamesproximity to solid surfaces, resulting in significant quenching due tolosses of heat as well as important reactive radicals. Mixing is limitedby the narrow channels, the low velocities, and hence the laminar flowsthat result only in molecular mixing of species for the preparation of acombustible mixture. Efforts to date have been unsuccessful in enhancingmixing in micro-fluidity devices. As a result, efforts to constructmicro-combustors have been largely limited to premixed flames to bypassthe issues of mixing and to focus on combustion stability. Stability,however, remains an issue even for premixed flames, largely due to poorfuel conversion.

Accordingly, there is a need to address the aforementioned deficienciesand inadequacies.

SUMMARY

The present disclosure provides a micro-mixer/combustor to mix fuel andoxidant streams into combustible mixtures where flames resulting fromcombustion of the mixture can be sustained inside its combustionchamber. The present design is particularly suitable for diffusionflames. In an aspect the present design mixes the fuel and oxidantstreams prior to entering a combustion chamber. The combustion chamberis designed in a way that does not allow excess pressure to build upwithin the combustion chamber, which build up can cause instabilities inthe flame. A restriction in the inlet to the combustion chamber from themixing chamber forces the incoming streams to converge while introducingminor pressure drop. In one or more aspects, a catalytic strip may bepositioned in the combustion chamber to initiate reactions and tosustain a flame without preheating.

Briefly described, one embodiment, among others, provides amicro-mixer/combustor comprising:

a fuel inlet and an oxidant inlet;

a mixing chamber downstream from the fuel and oxidant inlets, and incommunication with the fuel and oxidant inlets, designed to mix fuel andoxidant received from the fuel and oxidant inlets, the mixing chamberhaving walls that converge towards each other downstream from the fueland oxidant inlets, the walls forming a restriction at the downstreamend of the mixing chamber restricting the flow of fuel and oxidant outof the mixing chamber;

a combustion chamber downstream from the restriction and in fluidcommunication with the mixing chamber through the restriction, thecombustion chamber including walls that diverge from each other from therestriction, the combustion chamber being wider than the restriction atan end of the combustion chamber downstream from the restriction; and

an exhaust outlet downstream from the combustion chamber for exhaustingcombustion products from the combustion chamber.

In one or more aspects, heat from combustion products exhausted from thecombustion chamber may be used to provide heat to preheat at least oneof fuel passing through the fuel inlet channel, oxidant passing throughthe oxidant inlet channel, the mixing chamber, or the combustionchamber. In a non-limiting aspect heat from the combustion products mayprovide preheating to at least 100° C.

In one or more aspects, the micro-mixer/combustor includes an outer wallencasing the micro-mixer/combustor, the outer wall formed of a materialhaving a low thermal conductivity. In various aspects themicro-mixer/combustor may, but need not, be externally adiabatic.

In one or more aspects the micro-mixer/combustor may include an outerwall and two inner walls. The inner walls may be positioned inside ofthe outer wall and spaced apart from the outer wall. The inner walls mayfurther be positioned opposite and spaced apart from each other, theinner walls providing the walls forming the mixing chamber, restrictionand combustion chamber in the space between the inner walls. The spacingbetween the inner walls and the outer wall may form the exhaust outlet.At least one of the inner walls may be formed of a thermally conductivematerial allowing heat from combustion products passing through theexhaust outlet to be transferred through at least one of the inner wallsto provide heat to preheat at least one of fuel passing through the fuelinlet channel, oxidant passing through the oxidant inlet channel, themixing chamber, or the combustion chamber. The inner walls may bepositioned inside of the outer wall and spaced apart from the outer wallto form an exhaust outlet having at least two exhaust channels, anexhaust channel provided in the space between one of the inner walls andthe outer wall and a second exhaust channel provided in the spacebetween the second inner wall and the outer wall. At least one of theexhaust channels may be positioned on a side of an inner wall oppositeat least one of the mixing chamber, restriction or combustion chamber.

In one or more aspects, the micro-mixer/combustor may include acatalytic ignition strip in the combustion chamber. The catalyticignition strip may be a platinum strip or a piece of platinum positionedon or in the side of a wall forming the combustion chamber. The ignitionstrip may be positioned in the combustion chamber to initiate ignitionand to sustain a flame without preheating.

In another aspect, a method of mixing and combusting a fuel and anoxidant is provided including the steps of:

providing the micro-mixer/combustor such as described in one or moreaspects above, the micro-mixer/combustor including an outer wall and twoinner walls, the inner walls positioned inside of the outer wall andspaced apart from the outer wall, the inner walls further positionedopposite and spaced apart from each other, the inner walls providing thewalls forming the mixing chamber, restriction and combustion chamber,the spacing between the inner walls and the outer wall forming theexhaust outlet, the exhaust outlet including an exhaust channel formedin a space between the outer wall and at least one of the inner wallsand positioned on a side of the at least one inner wall opposite atleast one of the mixing chamber, restriction or combustion chamber,wherein the at least one inner wall is formed of a thermally conductivematerial allowing heat from combustion products passing through theexhaust channel to be transferred through the at least one inner wall toprovide heat to preheat at least one of fuel passing through the fuelinlet channel, oxidant passing through the oxidant inlet channel, themixing chamber, or the combustion chamber;

introducing fuel into the mixing chamber of the micro-mixer/combustorthrough the fuel inlet;

introducing oxidant into the mixing chamber of the micro-mixer/combustorthrough the oxidant inlet;

mixing the fuel and the oxidant in the mixing chamber;

passing the mixture of fuel and oxidant through the restriction into thecombustion chamber;

combusting the mixture of fuel and oxidant in the combustion chamber;

exhausting combustion products resulting from the combustion of themixture of fuel and oxidant out of the combustion chamber and throughthe exhaust channel, such that heat from the combustion products passingthrough the exhaust channel is transferred through the at least oneinner wall and into at least one of the mixing chamber, restriction orcombustion chamber to preheat the fuel, the oxidant and/or the mixtureof the fuel and oxidant in the micro-mixer/combustor.

In one or more aspects, the fuel, the oxidant and/or the mixture of thefuel and oxidant may be heated or preheated to at least 100° C. Theouter wall may be formed of a material having a low thermalconductivity. In various aspects the micro-mixer/combustor may, but neednot, be externally adiabatic.

Other systems, methods, features, and advantages of the presentdisclosure for a micro-mixer/combustor, in particular for diffusionflames, will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 depicts a 2D longitudinal cross-sectional view of an embodimentof a micro-mixer/combustor of the present disclosure.

FIG. 2 depicts a 3D perspective view of the micro-mixer/combustor ofFIG. 1.

FIGS. 3 a-c depict back, right side and front views of themicro-mixer/combustor of FIGS. 1 and 2.

FIG. 4 depicts simulated color contours of mass fraction of methane forinflows of methane and air in an embodiment of a micro-mixer/combustorof the present disclosure.

FIG. 5 depicts a sequence of images tracing a computed evolution oftemperature in the micro-mixer/combustor of FIG. 4 for various timesafter ignition.

FIG. 6 a depicts the computed steady-state temperature and selected massfractions for steady-state cases when the fuel and air inlet streamswere heated to 100° C. and 300° C.

FIG. 6 b depicts the computed contours of temperature and selectedspecies mass fractions for three cases of material properties within themicro-mixer/combustor of FIG. 4.

FIG. 7 a depicts the temperature contours for a case where the inlettemperatures are set at 100° C. and the internal material is aluminum asan internally conductive material.

FIG. 7 b depicts an embodiment of the micro-mixer/combustor as amilliburner with a gap size of 10 mm.

FIG. 7 c depicts color contours of temperature of embodiments of themilliburner of FIG. 7 b with a gap size of 4 mm comprised of fusedsilica and stainless steel.

FIG. 7 d depicts color contours of temperature for the milliburner ofFIG. 7 b with a gap size of 1.5 mm comprised of fused silica.

FIG. 8 a depicts an embodiment of the present disclosure incorporating acatalytic ignition strip in the combustion chamber.

FIG. 8 b depicts simulated temperature and mass fractions for theembodiment of FIG. 8 a.

DETAILED DESCRIPTION

Described below are various embodiments of the present systems andmethods for a micro-mixer/combustor. Although particular embodiments aredescribed, those embodiments are mere exemplary implementations of thesystem and method. One skilled in the art will recognize otherembodiments are possible. All such embodiments are intended to fallwithin the scope of this disclosure. Moreover, all references citedherein are intended to be and are hereby incorporated by reference intothis disclosure as if fully set forth herein. While the disclosure willnow be described in reference to the above drawings, there is no intentto limit it to the embodiment or embodiments disclosed herein. On thecontrary, the intent is to cover all alternatives, modifications andequivalents included within the spirit and scope of the disclosure.

The present disclosure is directed to a micro-mixer/combustor thatprovides suitable mixing of fuel and oxidant streams and stablecombustion of the mixed streams. In particular, in various aspects, itprovides a stable diffusion flame of the mixture of the fuel and oxidantstreams. In general, the micro-mixer/combustor includes separate inletsfor delivering fuel and oxidant to a mixing chamber. A restriction isprovided at an outlet from the mixing chamber downstream from the fueland oxidant inlets, the restriction leading to a combustion chamber. Thecombustion chamber includes one or more outlets to exhaust combustionproducts from the combustion chamber. In various aspects the mixingchamber is designed to gradually optimize the mixing of the fuel andoxidant streams prior to the mixture entering the combustion chamber.The combustion chamber may be designed to minimize build-up of thepressure of combustion products in the combustion chamber.

A non-limiting embodiment of our present micro-mixer/combustor isillustrated in FIGS. 1-3. FIG. 1 depicts a 2D longitudinalcross-sectional view of an aspect of our micro-mixer/combustor. In thisaspect, our micro-mixer/combustor 10 includes an outer wall 12 havingsides 14, 15, and 16. As depicted the external surfaces of sides 14, 15,and 16 form a generally U-shaped outer wall 12. The outer wall 12 neednot, however, have a generally U shaped configuration. Otherconfigurations may be possible. As seen by reference to one or moreexamples below, in one or more aspects our micro/mixer-combustor may besub-decimetric, meaning that the dimensions of outer wall 12, includingits sides, may have dimensions less than a decimeter.

The micro-mixer/combustor 10 further includes inner walls or baffles 22,26 positioned inside of the outer wall 12 and spaced inwardly and apartfrom the outer wall 12. A splitter wall 22 a is positioned inwardly andspaced apart from wall or baffle 22. The spacing between inner wall 22and splitter wall 22 a provides a channel 24 serving as a fuel inlet 25.As depicted in FIG. 1 inner wall 22 is generally linear having straightsides, though other configurations may be possible. Inner wall or baffle26 is spaced apart from and opposed to inner wall 22 and also spacedapart and separate from outer side wall 16. The spacing between innerwall or baffle 26 and splitter wall 22 a forms a gap that serves as anoxidant inlet channel 27.

Inner wall/baffle 26 includes surfaces 26 a-c spaced apart from butfacing inner wall 22, including a portion 26 a that narrows the spacingbetween walls 22 and 26 forming a restriction 32. The space between theinner walls/baffles 22 and 26 and between the restriction 32 and the endof the oxidant channel 27 closest to the restriction 32 forms a mixingarea or chamber 34 within the micro-mixer/combustor 10. On the oppositeside of restriction 32 the space between inner walls/baffles 22, 26 andbetween restriction 32 and sidewall 15 forms a combustion chamber 36 inwhich fuel and oxidant mixed in the mixing chamber 34 and passingthrough restriction 32 can be combusted. One or more exhaust channelsare provided to exhaust combustion products from the combustion chamber36. In the aspect depicted in FIG. 1, two exhaust channels 38 a, b aredepicted. Exhaust channel 38 a is formed in the space between outer wall14 and inner wall 22, and exhaust channel 38 b is formed in the spacebetween outer wall 16 and inner wall 26. It should be understood thatother arrangements for exhaust of the combusted materials can beprovided.

In FIG. 1, it can be seen that the oxidant inlet channel 27 is larger incross-section than the fuel inlet channel 25. In an aspect the size ofthe inlet streams can be proportioned close to the stoichiometric ratioof fuel/oxidant when the same inlet velocity is maintained for bothstreams. Mixing of fuel and oxidant occurs relatively quickly in themixing chamber 34 as the flow approaches the restriction 32, and perfectmixing can be achieved within the mixing chamber 34 downstream from theend of fuel channel 24 leading into the mixing chamber 34. Therestriction 32 allows the upstream fuel and oxidant streams to convergeand mix prior to entering the reaction chamber 36. In an aspect theinner surface 26 b of wall/baffle 26 between the oxidant inlet 27 andrestriction 32 converges toward wall 22, narrowing the spacing betweenwalls/baffles 22 and 26 forming restriction 32. In an embodiment innersurface 26 b of wall/baffle 26 can curve towards splitter wall 22 a andwall 22. In an embodiment, splitter wall 22 a can have a curved innersurface (opposite to and facing surface 26 b of wall/baffle 26) leadingto a point at its end closest to restriction 32. The inner curvedsurface of splitter wall 22 a and the inner curved surface 26 b ofwall/baffle 26 can cooperate to direct the flow of the oxidant into theflow of the fuel stream inducing mixing of the streams. The restriction32 introduces a pressure drop, though it can be relatively small.

In the aspect depicted in FIG. 1 the combustion chamber 36 is formed bydiverging opposed walls/baffles 22 and 26. In an aspect the combustionchamber 36 may have a generally triangular cross-section formed betweeninner wall 22 and the downstream portion 26 c of the inner surface ofwall/baffle 26. The cross-section of the combustion chamber 36downstream of the restriction 32 allows for gas expansion due to heatrelease and reduces pressure buildup within the combustion chamber 36.The widest point at the downstream end of the reaction chamber 36 leadsto an exhaust outlet for the combustion chamber. In an aspect theexhaust outlet includes two side channels 38 a, b for exhaustingcombustion products from the combustion chamber.

In various aspects exhaust channels 38 a, b can wrap around the body ofthe burner 10, overlapping inlet streams 25, 27 to provide heating orpreheating of the incoming fuel and oxidant gases and the mixing chamber34 by conduction of heat from the combustion products exiting exhaustchannels 38 a, b through walls/baffles 22 and/or 26. Surface 26 d ofwall/baffle 26 and the opposed inner surfaces 16 a and 16 b of outerwall 16 can form an exhaust channel 38 b designed to drive thecombustion products along the outlet channel 38 b such that the hotcombustion products travel closer to the cold reactants so that heatstored in the combustion products is used in an efficient way to preheatthe reactants by conduction through wall/baffle 26. In the non-limitingexample depicted in FIGS. 1-3 exhaust channel 38 b runs generallyparallel to surfaces 26 a-c, though this is not necessary. Similarlyheat in combustion products passing through exhaust channel 38 a can beused in an efficient way to preheat the reactants by conduction throughwall 22.

A 3D view of the micro-mixer/burner is presented in FIG. 2 and the back,right side and front views of the micro-mixer/burner are shown in FIGS.3 a, 3 b and 3 c, respectively. The flow channels (fuel channel 24,oxidant channel, mixing chamber 34, restriction 32, combustion chamber36 and outlet channels 38 a, 38 b are grooved within a block of lowthermal conductive material. The depth of the groove may depend on themanufacturing material. The depth of the groove is a important parameterthat can be related to the quenching distance below which the flamecannot be sustained. The flow channels are confined by back wall 17(shown) and a front wall opposite back wall 17 (cut away and not shown)which may serve as additional source of heat loss. However, in one ormore aspects back wall 17 and the front wall may be adiabatic. Thedistance between the inside surface 17 a of the back wall 17 and theinside surface of the front wall opposing surface 17 a is sometimesreferred to as the “gap size” of the micro-mixer/combustor 10.

Suitable fuels for use in our present micro-mixer/combustor includeorganic compounds typically used in or for combustion. The organiccompounds can include hydrocarbons, such as methane, ethane, propane,butane and propylene. Other suitable fuels include hydrogen and dimethylether. The fuel may be in gas or liquid phase in the storage container.However when entering the micro-mixer/combustor through the inlets 25and 27 the fuel is preferred to be in vapor phase. By changing thevelocities of the fuel and oxidant streams 25, 27 we can reach anymixture we want prior to entering the combustion chamber 36. Thereforethis design may provide the user full control over the stoichiometry ofany fuel/oxidant mixtures by adjusting the flow rates of the fuel andoxidant accordingly.

Suitable oxidants include those typically used in or for combustion, forexample oxygen and air. Other suitable oxidants include ozone, hydrogenperoxide, fluorine and chlorine; however air and oxygen are the safestand cheapest oxidants. Also, possible oxidants include mixtures ofoxygen and other inert gases such as nitrogen and argon. This mixingmethod can be utilized to control the temperature of the combustionprocess so that no material failure due to high temperatures isobserved.

Suitable materials of construction for our micro-mixer/combustor includematerials that would maintain the micro-mixer/combustor almostexternally adiabatic, thus minimizing if not preventing heat loss orheat transfer from the combustion chamber 36 outwardly through the outerwalls (including wall 12, back wall 17 and the front wall opposite backwall 17) and through inner walls/baffles 22 and 26 (26 c) that are incontact with the combustion chamber 36 that can lead to flame quenching.Preferred materials for the outer walls include materials having lowthermal conductivity. Suitable materials having a low thermalconductivity include ceramic materials such as quartz and fused silica.Other materials may be used, however, particularly if combined withthermal insulation provided on the outside of outer walls to minimize orprevent heat loss. Suitable materials are also materials that canwithstand high temperatures without degradation. Materials for innerwalls or baffles 22 and 26 can include any materials that can withstandthe temperatures of combustion also without degradation. The selectionof material for inner walls/baffles 22 and 26 may depend upon whetherheat conduction is to be provided from the exhaust streams 38 a, b toheat or preheat one or both of the inlet fuel and oxidant streams 25 and27 and/or mixing chamber 34. Further considerations for materials forthe present micro-mixer/combustor are materials that are surface treatedto minimize the quenching of radicals such as annealed polycrystallinealumina which is baked in a high temperature oxygen environment.

In one or more further aspects, a metal strip may be provided on or inan inner wall of the combustion chamber 26 to serve as an ignitionsource for igniting the fuel/oxidant mixture. As an example, a catalyststrip can be added along a portion of surface 17 a (FIG. 2) of innerwall 17 that constitutes a portion of the combustion chamber 36. Also, ametal strip may be provided along at least a portion of surface 26 c ofinner wall/baffle 26 that forms a portion of combustion chamber 36. Ametal strip may also be provided on or in at least a portion of theinner surface of wall 22 that forms combustion chamber 36 in conjunctionwith surface 26 c. As an example, a platinum strip can be provided alongat least a portion of a wall forming the combustion chamber 36 that willprovide ignition of a fuel+hydrogen/oxidant mixture entering thecombustion chamber 36. Platinum can serve to ignite such a mixture at orabove ambient temperature. By doing so no external ignition source isneeded. This may facilitate its implementation in practical systems.

Examples

In order to test out the present design, an exemplarymicro-mixer/combustor was simulated having overall external dimensionsof 2.5 mm for the length of outer wall 15 and 4.6 mm for the lengths ofouter walls 14 and 16 with a fuel inlet channel 24 having a width of0.08 mm and the oxidant inlet 27 having a width of 0.75 mm. Methane wasassumed as the fuel and pure air (21% oxygen and 79% nitrogen) as theoxidant. The size of the fuel and oxidant inlet streams was thusproportioned close to the stoichiometric ratio of methane/air when thesame inlet velocity is maintained for both inlet streams. Color contoursof the mass fraction of methane are shown for inflows of methane and airat 0.5 ms in 27° C. on the left side of FIG. 4. In this simulationmixing of methane and air occurs relatively quickly as the flow of thestreams approaches restriction 32. Perfect mixing of both streams isachieved within mixing chamber 34 downstream from fuel inlet channel 24,as is evident from the false contours shown on FIG. 4. The narrowestpassage at the neck of the restriction is 0.1 mm. The introduction ofthe restriction 32 allows the upstream methane and air streams toconverge and mix prior to entering the combustion chamber 36.

For the 2D burner configuration shown in this simulation, the volumeflow-rate of air is 9.375 times that of methane, leading to an overallstoichiometric composition. This is achieved just before the restrictioninlet to the combustion chamber 36 so that a mixture with Ø≈1 enters thecombustion chamber 36, as seen from the color contours of FIG. 4corresponding to a stoichiometric mass fraction of methane of ˜0.053filling the remainder of the mixing chamber 34. This burner design (or avariation thereof), particularly in regards to the fuel and oxidantinlets, mixing chamber, restriction and combustion chamber, is used herefor all subsequent calculations and is later extended to a 3Dconfiguration as described below. As can be seen variations includevariation in the exhaust channels 38 a, b.

Numerical Setup

The commercial Fluent 12 (Fluent 12, Ansys, 2009) CFD package was usedfor all calculations presented here. The Tri-pave meshing scheme isadopted, which allows us to control the aspect ratio and refine the meshwhere needed. The computed species concentrations as well as temperatureprofiles at various locations in the domain are compared for variousgrid sizes to ensure that that a grid-independent solution is presented.The flow, reactants, and energy equations are solved first so as toprovide a good starting point for the more complicated case wheregaseous reactions dominate the solution. The results presented hereinare obtained from the non-iterative time advancement unsteady state partof the solver for a time step of 1.0 μs, and then the steady laminarsolver is turned on to ensure that the solution is fully converged. Asecond-order discretization scheme has been utilized for all theequations solved, and the under-relaxation parameters have been modifiedslightly to help converge and stabilize the solution.

The Smooke mechanism (Smooke et al., 1986) with the correspondingthermo-dynamic database file was used for the volumetric reactions. TheGRI2.11 transport database file was used with the selected mechanisms toaccount for chemical reactions with mass, heat, and thermal conductivitydiffusivity. Since the flow is laminar, the full multicomponentdiffusion model must be enabled for the careful treatment of chemicalspecies diffusion in the species transport and energy equations. Thermaldiffusion is solved as well, and detailed gas chemistry is implementedusing the ISAT algorithm where the ISAT error tolerance of 1e-6 wasused.

Mixing Issues

In order to understand the mixing mechanism of fluid flow inmicro-combustors, we examined the conservation equations of momentum,energy, and species normalized by the characteristic length and otherrelevant parameters of the device (Fernandez-Pello, 2002) as shownbelow:

$\begin{matrix}{{{\frac{l_{c}}{t_{c}u_{c}}\frac{\partial\overset{\_}{u}}{\partial\overset{\_}{t}}} + {\overset{\_}{u}\frac{\partial\overset{\_}{u}}{\partial\overset{\_}{x}}}} = {{{- \frac{p_{c}}{\rho_{c}u_{c}^{2}}}\frac{\partial\overset{\_}{p}}{\partial\overset{\_}{x}}} + {\frac{1}{Re}\overset{\_}{v}\frac{\partial^{2}\overset{\_}{u}}{\partial\overset{\_}{x^{2}}}} + \frac{g\; l_{c}}{u_{c}^{2}}}} & (1) \\{{{\frac{l_{c}}{t_{c}u_{c}}\frac{\partial\overset{\_}{T}}{\partial\overset{\_}{t}}} + {\overset{\_}{u}\frac{\partial\overset{\_}{T}}{\partial\overset{\_}{x}}}} = {{\frac{1}{Pe}\overset{\_}{\alpha}\frac{\partial^{2}\overset{\_}{T}}{\partial\overset{\_}{x^{2}}}} + {{Da}\frac{Q}{{\overset{\_}{C}}_{{pT}_{c}}}{\overset{\_}{\overset{.}{w}}}^{''}}}} & (2) \\{{{\frac{l_{c}}{t_{c}u_{c}}\frac{\partial{\overset{\_}{y}}_{i}}{\partial\overset{\_}{t}}} + {\overset{\_}{u}\frac{\partial{\overset{\_}{y}}_{i}}{\partial\overset{\_}{x}}}} = {{\frac{1}{LePe}\overset{\_}{D}\frac{\partial^{2}{\overset{\_}{y}}_{i}}{\partial\overset{\_}{x^{2}}}} + {{Da}\frac{1}{y_{ic}}{\overset{\_}{\overset{.}{w}}}^{''}}}} & (3)\end{matrix}$

The above equations are derived for a continuum fluid, which is still avalid assumption for the flow considered here. Reference should be madeto the Nomenclature (Appendix A) for explanatory details about thesymbols. Equation (1) describes the momentum of the flow normalized bythe parameters of the device, where the first term on the left-hand sideis the unsteady acceleration, the second term on the left-hand side isthe convective acceleration, the first term on the right-hand side isthe pressure gradient, the second term on the right-hand side is theviscosity effect, and the third term on the right-hand side is thegravity body force.

Details about a number of dimensionless numbers are included: Reynoldsnumber, Péclet number, Damköhler number, and Lewis number. The Pécletnumber, Pe, is defined as the ratio of the rate of advection of the flowto the rate of diffusion:

$\begin{matrix}{{Pe} = \frac{l_{c}u_{c}}{\alpha_{c}}} & (4)\end{matrix}$

The Damköhler number, Da, is defined as characteristic mixing time orthe ratio of time it takes for a fluid to travel a certaincharacteristic distance to the time it takes for the chemical reactionto complete:

$\begin{matrix}{{Da} = \frac{{\overset{.}{w}}_{c}^{''}l_{c}}{\rho_{c}u_{c}}} & (5)\end{matrix}$

The Lewis number, Le, is defined as the ratio of thermal diffusivity tomass diffusivity:

$\begin{matrix}{{Le} = \frac{\alpha_{c}}{D_{c}}} & (6)\end{matrix}$

In Equations (1)-(3), the diffusive terms are multiplied by the inverseof Reynolds number in Equation (1) and the inverse of Péclet number inEquations (2) and (3). Therefore, in the case of a large mixing system,viscous and diffusive terms are small relative to advection terms. Asthe characteristic lengths of the components become smaller (such is thecase with microburners), the values of Reynolds and Peclet numbersdecrease since the flow laminarizes and the advection terms becomenegligible. Mixing in such small devices is harder to achieve since itis largely driven by molecular processes, and hence a judicious designof the mixing streams is needed. The next section presents results fortwo microburner designs: the first leads to inadequate mixing, while thesecond is an almost perfect mixer.

Combustion in 2D

Solutions are now presented for three reacting cases in 2D with thefollowing conditions: (i) an adiabatic, nonconductive burner with inletstreams at 27° C. leading to an extinguishing flame, (ii) an adiabatic,nonconductive micro-mixer/combustor (burner) with inlet streams at 100°C. and 300° C. leading to a stable flame, and (iii) an adiabatic burnerwith internal conduction and inlet streams at 300° C. leading to astable flame. In this latter case, the outer walls 14-16 of the burnerare adiabatic, but the inner walls/baffles 22 and 26 are conductive(non-adiabatic), enabling heating of the inlet channels 25 and 27 andthe mixing and combustion chambers 34 and 36.

1) Extinguishing Case: Adiabatic, Nonconductive with Inlet at 27° C.

A sequence of images tracing the computed evolution of temperature inthe 2D micro-mixer/combustor 10 is shown in false color contours in FIG.5. Results are shown for various times after ignition for the case whereboth methane and air streams enter the burner at a velocity of 0.5 m/sand a temperature of 27° C. First, a solution for the non-reacting caseis obtained showing almost complete mixing before the restriction 32, asshown in FIG. 4. This is used as a starting point for the reacting case.Ignition is initiated by introducing in the first iteration a hot patchthat has a temperature of 2500 K and dimensions of 0.66 mm×0.85 mmcentered in the middle of the triangular combustion chamber 36downstream of the restriction 32, as shown in the first plate for a timeof 10 μs. The case considered here is fully adiabatic.

It is evident from the sequence shown in FIG. 5 that the flame cannot besustained for these inlet conditions and extinguishes with the initialcombustion products, washing off through the left and right exhaustoutlets 38 a, b. It can be seen from the images at 100 μs that the flameis initiated at the hot spot and propagates back to consume the unburnedmixture of methane and air. The flame extinguishes, as indicated by thedecreased peak temperature, and the combustion products gradually startto flush out of the combustion chamber 36 through the side exhaustoutlets 38 a, b as shown from the contours of the 1 to 6 ms. Thecombustion products continue to exit the combustion chamber 36 until thefully non-reacted solution is recovered at times >8 ms.

2) Burning Case: Adiabatic, Nonconductive with Inlet at 100° C. and 300°C.

Given that the previous case extinguished, the inlet mixture temperatureis increased here from 27° C. to 100° C. and then to 300° C. for thesame adiabatic case, and a stable flame was obtained in both cases,albeit for a different position within the chamber. The computedsteady-state temperature and selected species mass fractions (OH, O₂,and CO) for the steady-state case are shown in FIG. 6 a for both casesof 100° C. and 300° C. Note that the solid sections of the burner,namely outer wall 12 and inner walls/baffles 22 and 26, which areassumed here to have zero conductivity, are shown to be at the sametemperatures of the entering mixture since no heat transfer is allowedhere. It is evident from these results that, as the temperature of theincoming mixture increases, the peak mass fraction of OH increases andthe flame front moves upstream closer to the restriction 32. This isconsistent with the corresponding increase in flame speed at the hotterconditions. Oxygen is fully consumed in both cases, and CO forms righton the reaction zone in the combustion chamber 36 and gets consumedquickly to form CO₂ (not shown here), which exists in higher quantitiesfor the hotter inlets as a result of stronger reaction zone.

3) Burning Case: Adiabatic with Internal Heat Conduction and Inlet at300° C.

In this third case, our calculations now allow for heat conductionwithin the inner core of the burner but no external heat losses throughouter wall 12 so that the overall burner remains adiabatic. In this caseheat from the combustion products exiting through exhaust channels 38 a,b is allowed to pass through inner walls/baffles 22 and 26 to the inletchannels to allow preheating of the methane and air inlet streams 25 and27 and also the mixing chamber 34. Results are shown here for threematerials, namely aluminum, steel, and fused silica for which relevantproperties are shown in Table 1. For these materials, a flame cannot bestabilized when the temperature of the mixture is 27° C., socalculations are shown here for a temperature inlet of 300° C. where astable flame is obtained.

FIG. 6 b shows the computed contours of temperature and selected speciesmass fractions (OH, O₂, and CO) for three cases where the materialproperties within the core of the burner (namely, the innerwalls/baffles 22 and 26) change from aluminum to steel to fused silica.It is clear that the flame stability, as marked by the peak temperatureand the maximum levels of OH formed, improves with the decreasingconductivity of the material. With fused silica, which has a lowconductivity of 1.3 W/m·K, the flame front has actually moved upstreamof the neck, and some reaction has occurred at the tip of the splitterplate 22 a (FIG. 1) separating the fuel and air streams where thetemperature is 2100 K, and some CO as well as OH have formed. Aluminumand steel are very similar in terms of flame temperature andcomposition, but both are significantly different than fused silica.When using fused silica, the flame becomes hotter in the initial stages,and peak temperatures of 2700 K are observed at the restriction 32, theflame passes the neck and starts burning on top of the splitter 22 a asshown in FIG. 6 b where richer methane/air mixtures exist (see FIG. 4).This explains the lower flame temperature when using fused silicacompared to aluminum and steel, where the flame sits at the neck andalmost stoichiometric methane/air mixtures are burned.

TABLE 1 Material properties as adopted in the current calculationsDensity Specific heat Thermal conductivity Material (kg/m³) (Cp) (J/kg ·K) (W/m · K) Aluminum 2719 871 202.4 Steel 8030 502.48 16.27 Fusedsilica 7203 740 1.3

Combustion in 3D

The 3D version of micro-mixer/combustor is slightly modified from its 2Dversion, where the products travel closer to the reactants for a longertime to allow better heat exchange between hot products and reactants.The micro-mixer/combustor is sandwiched between two solid plates (wall17) that are 1 mm apart. Only half of the domain is modeled due tosymmetry along the third dimension, and the domain is meshed using85,000 triangular cells and ran for mixing only; the results for mixingwere the same as for the 2D case. FIG. 7 a shows the temperaturecontours for a case, where the inlet temperatures of fuel and air areset to T_(jet)=100° C.; the velocities of both stream are 0.5 m/s andthe material used here is aluminum. Only internal heat transfer isallowed here, so the burner is externally adiabatic.

As can be observed from the computed temperature contours shown in FIG.7 a, the flame is stable and the reaction zone sits close to therestriction. The internal heat exchange with the combustion products hasallowed the entering reactants to heat even further, reaching atemperature of 1400 K. Further investigation on the 3D micro-burner willbe carried out later to include a catalyst to assist ignition andstabilize the flame inside the burner.

Combustion in 3D (Scaled-Up Version)

In the micro-mixer/combustor, and for the non-adiabatic case, a stableflame cannot be sustained due to heat losses through the side and backwalls exceeding the heat generated by the flame. Various scaled-upversions of micro-mixer/combustor are tested here with various materialsand thermal conductivities in an attempt to find a threshold beyondwhich a stable flame is obtained. The external heat transfer coefficientis 20 W/m²·K, and a free stream temperature of 300 K. The velocities ofthe fuel and air streams are 0.5 m/s. FIG. 7 b shows the temperaturecontours for a milliburner with a 10 mm gap size between the two solidplates (wall 17) using steel and fused silica. This corresponds to ascaling-up of 10 times in all three dimensions of themicro-mixer/combustor (the overall external dimensions being 25 mm (2.5cm) for outer wall 15 and 46 mm (4.6 cm) for outer walls 14 and 16; theoverall external dimensions thus being sub-decimetric).

As can be noticed from FIG. 7 b the flame is stabilized even when heattransfer is allowed from all the burner walls and as the thermalconductivity decreases, the temperature increases and the flame frontshifts closer to the neck. However, the material with low thermalconductivity (fused silica) has a disadvantage of having local hot spotsthat might lead to material failure. These hot spots on the back surfaceof the burner disappear when a material with higher conductivity (suchas steel) is used, as can be seen from FIG. 7 b.

The effect of the gap size between the two solid plates (wall 17) wasinvestigated by decreasing the distance between the constraining platesto 4 mm while keeping the thickness of the covering plates (wall 17)unchanged (2 mm) so the conduction heat loss remains constant. As thegap size decreases, hence the surface-to-volume ratio increases, moreheat is lost from the flame, which moves up as a result of the lowerlaminar flame speed. When the gap size is reduced to 4 mm (see FIG. 7 c)(instead of 10 mm), the flame is stabilized further down-stream closerto the exhaust vents. When using steel, the flame is sitting almost atthe top of the combustion chamber and is likely to extinguish with aslight increase in heat losses. The local hot spot that might causematerial failure still exists for the low conductive material such asfused silica.

Decreasing the gap size between the two solid plates (wall 17) furtherto 1.5 mm, which is lower than the quenching distance of methane/airmixture at ambient temperature (2 mm), causes a loss of the flame, evenwhen using fused silica, because of the higher heat losses. Preheatingthe fuel and air streams to 100° C. is found to be necessary for allburner material used here (fused silica) when the gap size is 1.5 mm andno catalytic ignition strip is present. However, when the gap size islarger, for example 4 mm or larger as in the prior examples, then nopreheating is required even in the absence of a catalytic ignitionstrip.

FIG. 7 d shows color contours of temperature for the fused silica burnerwith a gap size between the two solid plates (wall 17) of 1.5 mm. Theflame is stabilized closer to the exhaust ports 38 a, b, but no hotspots are observed with a maximum temperature of 1000° C. at the innerside of the back plate (surface 17 a).

As can be seen from the foregoing the present disclosure provides adesign of a micro-mixer/combustor that mixes separate fuel and oxidantstreams and stabilizes a diffusion flame. An optimum design has beenachieved to perfectly mix the fuel and air to stoichiometric mixturesbefore entering the combustion chamber 36. Flame stabilization insidethe micro-mixer/combustor is numerically achieved using both 2D and 3Dgeometries. It was found that for a totally adiabaticmicro-mixer/combustor, a flame could be sustained if the incoming gasesare heated to at least 100° C., and as the mixture temperatureincreases, the flame moves upstream because of the increased laminarflame speed. When heat transfer is allowed within the reactor, withoutallowing heat transfer to the surroundings, the flame becomes morestable and stabilized further upstream within the combustion chamber.Further decreasing the thermal conductivity results in a flame travelingbeyond the neck and sitting on top of the splitter 22 a. 3D simulationsback up the 2D calculations for the externally adiabatic cases. Studieson a scaled up version of the micro-burner and the effect of the gapsize showed without preheating the fuel/air streams a gap size more than1.5 mm is required, and as the gap size decreases the flame weakens dueto higher heat losses as a result of higher surface to volume ratio.

Combustion in 2D (Micro-Burner with Catalytic Ignition)

The previous discussion about the 2D micro-burner showed that the flamecannot be stabilised if any heat is lost from that particular 2D burner.Therefore, as illustrated for example in FIG. 8 a, the micro-burner maybe turned into a Catalytically Stabilised (CST) micro-burner by adding acatalytic ignition strip 40, for example a platinum strip. The additionof the catalytic strip, such as platinum, benefits from a higher surfaceto volume ratio since the catalytic surface acts as a heat source not aheat sink. Also, since CH₄ or any of the hydrocarbons do not ignite onplatinum without an external heat source, hydrogen is used within thefuel mixture to help the ignition of the fuel/air mixture on platinum.Hydrogen self-ignites on platinum at ambient temperatures, as reportedby (Deutschmann et al. 1996). The consumption of hydrogen on theplatinum surface provides enough heat to initiate the ignition of otherhydrocarbons that are mixed with the hydrogen as a fuel. An embodimentof a catalytic platinum plate that is inserted into the combustionchamber 36 of the micro-burner is shown in FIG. 8 a.

In a simulated example, a volume of 60% H₂ and 40% CH₄ is fed into thefuel inlet at 27° C. and 0.447 m/s. The oxidant used is air and is fedat 27° C. and 0.5 m/s. If the fuel and air mix perfectly within themicro-burner, an equivalence ratio of 0=0.5 will be achieved.

When steel is used as a heat conducting material in the micro-burner,and chemical reactions are enabled, it is observed reactions at theplatinum plate start and result in higher temperatures recorded on themonitor points next to the plate. However, the temperatures on themonitor points start to decrease until they reach ambient temperatures.The reason for this is because the heat generated by the platinum stripconsuming H₂ is conducted through the wall 22, as indicated by thearrows in FIG. 8 a, and the heat loss terminates the surface reactions.Low thermal conductive materials, such as fused silica, are simulatedand the reaction is sustained at the platinum plate as shown in FIG. 8b. As can be observed from FIG. 8 b, the temperatures are much lower(850K) and the reactions that take place purely on the platinum surfaceproduce a negligible amount of OH and CO at the leading edge of theplate.

It should be emphasized that the above-described embodiments are merelyexamples of possible implementations. Many variations and modificationsmay be made to the above-described embodiments without departing fromthe principles of the present disclosure. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

REFERENCES

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1. A micro-mixer/combustor comprising: a fuel inlet and an oxidantinlet; a mixing chamber downstream from the fuel and oxidant inlets, andin communication with the fuel and oxidant inlets, designed to mix fueland oxidant received from the fuel and oxidant inlets, the mixingchamber having walls that converge towards each other downstream fromthe fuel and oxidant inlets, the walls forming a restriction at thedownstream end of the mixing chamber restricting the flow of fuel andoxidant out of the mixing chamber; a combustion chamber downstream fromthe restriction and in fluid communication with the mixing chamberthrough the restriction, the combustion chamber including walls thatdiverge from each other from the restriction, the combustion chamberbeing wider than the restriction at an end of the combustion chamberdownstream from the restriction; and an exhaust outlet downstream fromthe combustion chamber for exhausting combustion products from thecombustion chamber.
 2. The micro-mixer/combustor of claim 1, whereinheat from the combustion products is used to provide heat to preheat atleast one of fuel passing through the fuel inlet channel, oxidantpassing through the oxidant inlet channel, the mixing chamber, or thecombustion chamber.
 3. The micro-mixer/combustor of claim 1, furthercomprising an outer wall encasing the micro-mixer/combustor, the outerwall formed of a material having a low thermal conductivity sufficientto reduce heat loss and sustain a combustion flame in the combustionchamber.
 4. The micro-mixer/combustor of claim 2, wherein heat from thecombustion products provides preheating to at least 100° C.
 5. Themicro-mixer/combustor of claim 1, further including a catalytic ignitionstrip in the combustion chamber.
 6. The micro-mixer/combustor of claim5, wherein the catalytic ignition strip is a platinum strip.
 7. Themicro-mixer/combustor of claim 1, further comprising an outer wall andtwo inner walls, the inner walls positioned inside of the outer wall andspaced apart from the outer wall, the inner walls further positionedopposite and spaced apart from each other, the inner walls providing thewalls forming the mixing chamber, restriction and combustion chamber,the spacing between the inner walls and the outer wall forming theexhaust outlet.
 8. The micro-mixer/combustor of claim 7, wherein atleast one of the inner walls is formed of a thermally conductivematerial allowing heat from combustion products passing through theexhaust outlet to be transferred through the at least one inner wall toprovide heat to preheat at least one of fuel passing through the fuelinlet channel, oxidant passing through the oxidant inlet channel, themixing chamber, or the combustion chamber.
 9. The micro-mixer/combustorof claim 8, wherein the inner walls are positioned inside of the outerwall and spaced apart from the outer wall to form the exhaust outlethaving at least two exhaust channels, an exhaust channel provided in thespace between one of the inner walls and the outer wall and a secondexhaust channel provided in the space between the second inner wall andthe outer wall.
 10. The micro-mixer/combustor of claim 9, wherein atleast one of the exhaust channels is on a side of an inner wall oppositeat least one of the mixing chamber, restriction or combustion chamber.11. The micro-mixer/combustor of claim 1, wherein themicro-mixer/combustor provides a diffusion flame.
 12. A method of mixingand combusting a fuel and an oxidant, comprising: providing themicro-mixer/combustor of claim 1, the micro-mixer/combustor including anouter wall and two inner walls, the inner walls positioned inside of theouter wall and spaced apart from the outer wall, the inner walls furtherpositioned opposite and spaced apart from each other, the inner wallsproviding the walls forming the mixing chamber, restriction andcombustion chamber, the spacing between the inner walls and the outerwall forming the exhaust outlet, the exhaust outlet including an exhaustchannel formed in a space between the outer wall and at least one of theinner walls and positioned on a side of the at least one inner wallopposite at least one of the mixing chamber, restriction or combustionchamber wherein the at least one inner wall is formed of a thermallyconductive material allowing heat from combustion products passingthrough the exhaust channel to be transferred through the at least oneinner wall to provide heat to preheat at least one of fuel passingthrough the fuel inlet channel, oxidant passing through the oxidantinlet channel, the mixing chamber, or the combustion chamber;introducing fuel into the mixing chamber of the micro-mixer/combustorthrough the fuel inlet; introducing oxidant into the mixing chamber ofthe micro-mixer/combustor through the oxidant inlet; mixing the fuel andthe oxidant in the mixing chamber; passing the mixture of fuel andoxidant through the restriction into the combustion chamber; combustingthe mixture of fuel and oxidant in the combustion chamber; exhaustingcombustion products resulting from the combustion of the mixture of fueland oxidant out of the combustion chamber and through the exhaustchannel, such that heat from the combustion products passing through theexhaust channel is transferred through the at least one inner wall andinto at least one of the mixing chamber, restriction or combustionchamber to preheat the fuel, the oxidant and/or the mixture of the fueland oxidant in the micro-mixer/combustor.
 13. The method of claim 12,wherein the combustion provides a stable diffusion flame.
 14. The methodof claim 12, wherein the fuel, the oxidant and/or the mixture of thefuel and oxidant is heated to at least 100° C.
 15. The method of any ofclaim 12, wherein the outer wall is formed of a material having a lowthermal conductivity sufficient to reduce heat loss and sustain acombustion flame in the combustion chamber.
 16. Themicro-mixer/combustor of claim 2, further comprising an outer wallencasing the micro-mixer/combustor, the outer wall formed of a materialhaving a low thermal conductivity sufficient to reduce heat loss andsustain a combustion flame in the combustion chamber.
 17. Themicro-mixer/combustor of claim 2, further comprising an outer wall andtwo inner walls, the inner walls positioned inside of the outer wall andspaced apart from the outer wall, the inner walls further positionedopposite and spaced apart from each other, the inner walls providing thewalls forming the mixing chamber, restriction and combustion chamber,the spacing between the inner walls and the outer wall forming theexhaust outlet.
 18. The micro-mixer/combustor of claim 3, furthercomprising an outer wall and two inner walls, the inner walls positionedinside of the outer wall and spaced apart from the outer wall, the innerwalls further positioned opposite and spaced apart from each other, theinner walls providing the walls forming the mixing chamber, restrictionand combustion chamber, the spacing between the inner walls and theouter wall forming the exhaust outlet.
 19. The method of claim 14,wherein the outer wall is formed of a material having a low thermalconductivity sufficient to reduce heat loss and sustain a combustionflame in the combustion chamber.